In conventional audio systems, a channel-based approach is employed. Each channel may for example represent the content of one speaker or one speaker array. Possible coding schemes for such systems include discrete multi-channel coding or parametric coding such as MPEG Surround.
More recently, a new approach has been developed. This approach is object-based, which may be advantageous when coding complex audio scenes, for example in cinema applications. In system employing the object-based approach, a three-dimensional audio scene is represented by audio objects with their associated metadata (for instance, positional metadata). These audio objects move around in the three-dimensional audio scene during playback of the audio signal. The system may further include so called bed channels, which may be described as signals which are directly mapped to certain output channels of for example a conventional audio system as described above.
A problem that may arise in an object-based audio system is how to efficiently encode and decode the object audio signals and preserve the quality of the coded signal. A possible coding scheme includes, on an encoder side, means for creating a downmix signal comprising a number of channels derived from the audio objects and bed channels, and means for generating side information which facilitates reconstruction of the audio objects and bed channels on a decoder side.
MPEG Spatial Audio Object Coding (MPEG SAOC) describes a system for parametric coding of audio objects. The system sends side information, i.e. an upmix matrix, describing the properties of the objects by means of parameters such as level difference and cross correlation of the objects. These parameters are then used to control the reconstruction of the audio objects on a decoder side. This process can be mathematically complex and often has to rely on assumptions about properties of the audio objects that are not explicitly described by the parameters. The method presented in MPEG SAOC may lower the required bit rate for an object-based audio system, but further improvements may be needed to further increase the efficiency and quality as described above.
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